1. Field of the Invention
The present invention is directed to an electronic navigation device. More particularly, the present invention is directed to a handheld electronic navigation device, having combined in a single handheld housing a GPS unit, an electronic compass, and an altimeter.
2. Description of the Related Art
Handheld navigation devices are well known, widely used, and are becoming increasingly popular. For example, GPS units, which have the ability to calculate a position and display map, route, and location information are becoming increasingly popular. Conventional GPS units also have the ability to calculate a user""s heading, or direction of travel, by determining the movement of a user over time. However, conventional GPS units are unable to determine heading or direction information when the user of the GPS device is not traveling at a sufficient speed, typically in the order of 4 mph or greater.
Accordingly, an outdoor enthusiast (such as a hiker or biker), being often at a standstill or traveling less than a sufficient speed, must typically carry additional instruments, such as a compass, in order to instantaneously determine direction or heading information. Furthermore, it is often additionally desired by such enthusiasts to have knowledge of altitude information. Retaining this information requires yet an additional instrument, such as an altimeter. Carrying multiple such devices is particularly cumbersome when hiking or biking.
In an attempt to overcome the foregoing, one available product combines a GPS unit and a barometric pressure sensor in the same housing. However, the pressure sensor is used only to augment GPS derived altitude information, not as a separate indicator of altitude. Another available product, the XL1000 by SILVA combines a GPS unit and an electronic compass, but not an altimeter.
Additionally, it would be desirable in a navigation unit to include extensive mapping information, and provision for uploading even additional map information as desired. Furthermore, it would be desirable for any such unit to have convenient options for calibrating the altimeter. In this regard, it is well known to calibrate an altimeter with known altitude information input by a user. It is also known, such as in a product known as the xe2x80x9cFreestyle Altimeter Watchxe2x80x9d, to permit storage in a memory of two known altitudes, such that when a user is in one of the two locations, the correspondingly stored altitude information may be recalled for calibrating the altimeter. While useful, this approach is limited to a pair of locations and, as with other known approaches, requires user input.
Accordingly, the need exists for an improved navigation unit. Particularly, the need exists for a portable navigation unit having GPS, compass, and altimeter outputs. The present invention fills the foregoing identified needs, and other needs, while overcoming the drawbacks of the prior art.
It is an object of the present invention to provide an improved navigation unit.
It is an object of the present invention to combine prior art navigation units of various types into a single unit capable of individually or simultaneously displaying navigation information for a variety of types of single display.
It is a further object of the invention to provide unique options for calibrating an altimeter.
These and other objects are achieved by a portable, handheld navigation unit having an internal processor. Connected to the processor are, at a minimum, an input (such as a keypad), a display, a memory, a tilt sensor, magnetic sensor, a barometric pressure sensor, and a GPS receiver, which also connects to an antenna, such as a GPS patch antenna. These components, along with a power supply (such as batteries) are housed within a plastic housing sized to be handheld. As will be understood and appreciated, the input and display are accessible at an exterior of the housing, in a conventional manner.
The navigation device of the present invention serves as a GPS unit, in that GPS signals from a plurality of satellites may be received by the GPS receiver, such that the processor calculates position information based upon the received signals. Additionally, map information stored in memory may be retrieved and displayed on the display along with the calculated position information. Additionally, as will be understood and appreciated, the memory may include stored route, forward path, or backward path information, which may be recalled for display on the display. The conventional use and operation of GPS units is well known, and need not be further described.
Additionally, the navigation device of the present invention serves as an altimeter. In particular, the barometric pressure sensor measures barometric pressure and provides the sensed barometric pressure information to the processor. The processor, utilizing stored software, then converts the measured pressure into an altitude, which may be displayed to the user on the display. The conversion of barometric pressure to altitude may be accomplished in any desired and conventional manner. For example, a lookup table may be provided in the memory, where the table contains altitude information corresponding to known barometric pressures. Thus, an altitude corresponding with a sensed barometric pressure may be retrieved from memory and displayed on the display. Alternatively, the preferably, altitude (or elevation) may be calculated using a known equation.
Because barometric pressure in a particular area changes with changing weather, it is necessary for the navigation device of the present invention to be periodically calibrated. In accordance with an aspect of the invention, calibration is accomplished by supplying a known altitude in a particular area to the instrument software so that a correction factor correlating current pressure to known altitude can be generated. In the absence of such a calibration, any change in pressure will be perceived by the processor as a change in altitude, even if it is the result of a change in weather (such as a cold front passing through the particular area in which the device is being used).
In accordance with the principles of the present invention, there are three manners in which the known altitude can be supplied to the instrument software of the navigation device of the present invention for calibration purposes. The first manner of calibration involves calibration by the user. In this case, the user of the navigation device places the device into a calibration mode utilizing the input, and enters the current, known altitude. The processor then generates a correction factor corresponding to the entered altitude information.
In the absence of user input data, the instrument software of the navigation device of the present invention may obtain altitude information from a topographical map stored in the memory and/or displayed on the display. When this manner of calibration is utilized, the GPS unit has acquired a sufficient number of satellite signals, and has determined a position of the device. A map corresponding to the area in which the navigation device is being used, is retrieved from memory and displayed on the display. Thus, with knowledge of the position of the GPS unit relative to the stored topographical map, the altitude is determined since that information is provided in the topographical map information.
In the event a topographical map is not stored in memory and/or displayed on the display of the navigation device (or in the event altitude information corresponding to the location of the unit is not stored in memory), and the user has not entered an altitude using the input of the device, the necessary correction factor can be calculated by the processor using GPS-derived altitude. In this regard, and as will be readily appreciated, the position of a GPS unit in three dimensions may be determined when at least four satellite signals are received and processed. While GPS-derived altitude will typically not be as accurate as the input of the known altitude, or even as accurate as that provided on a topographical map, GPS does nevertheless provide a fairly accurate method of calibrating the navigation device. In particular, using GPS-derived altitude allows for complete xe2x80x9cauto-calibrationxe2x80x9d of the navigation device, thus relieving the user of retrieving and/or displaying topographical map information or inputting a known altitude.
In accordance with an additional aspect of the present invention, the processor determines a difference between an altitude derived from barometric pressure information and a GPS derived altitude. When that difference is greater than a preselected amount, the processor determines that there has been no user input data entered, or that the user input data that has been entered is inaccurate, thus causing the processor to either (a) notify the user of the inaccuracy, or (b) rely upon topographical map information in the event it has been retrieved or displayed, or lastly rely upon GPS-derived altitude information in the absence of topographical map information.
Additionally, the presence of pressure sensor information and topographical maps permits the processor to determine vertical ascent/descent over a specified time period, rates of ascent/descent as the user travels or moves, as well as elevational profiles of user-selected routes stored in memory. Such information will be particularly useful to an individual, such as a hiker, biker, or hunter, in evaluating whether his or her intended path is too strenuous, or too easy, for his or her intended excursion.
As stated, the unique navigation device of the present invention also includes a compass. In accordance with the principals of the present invention, compass information is provided through a three-axis magnetic sensor and a two-axis tilt sensor. The three-axis magnetic sensor measures the strength of the earth magnetic field in each of the x, y, and z directions at the location of the navigation device, while the two-axis tilt sensor measures the orientation of the unit in pitch and roll as referenced to a known reference, such as the gravity vector (locally leveled tangent plane). The processor, and particularly the instrument software utilized by the present invention, takes this information and calculates a heading based upon the sensed magnetic field strengths and tilt information. This tilt information is used to reference the magnetic field data back to a level platform, so that accurate headings can be obtained. In this regard, as will be appreciated, without knowledge of tilt, the sensed x and y axis magnetic information cannot be transformed back to a level orientation. In order for accurate headings to be computed, the x and y sensor information must be referenced to a frame that is level, or at least substantially level, with the earth""s surface at the location of the navigation device. The transformation of the sensed magnetic field strengths and tilt information to a heading is accomplished in a conventional manner, such as set forth in xe2x80x9c3-D Vector Processing of Magnetometer and Inclinometer Dataxe2x80x9d, by David Gibson Cave and Karst Sciencexe2x80x9d, Vol. 23, No. 2, October, 1996, pages 71-76; White Paper. xe2x80x9cNongimbaled Solid-State Compassxe2x80x9d by Olson et al., Honeywell Technology Center, Minneapolis, Minn.; and xe2x80x9cAerospace Avionics Systems: A Modem Synthesisxe2x80x9d by George Siouris, Academic Press, 1993, page 35, each of which is incorporated herein by reference.
The addition of the magnetic and tilt sensors utilized by the present invention allows for a compass setting to be displayed at any time, with the unit in a wide variety of orientations, even when the user is standing still and moving at a slow speed. The addition of these sensors also allows for a moving map display to be realized when the user is standing still. In this regard, conventional GPS navigational devices already provide moving map displays, but only when the unit is in motion and the GPS heading information is available. Thus, it will be appreciated that, with the present invention, even when a user is standing still, a heading may be determined, and that heading may be utilized to xe2x80x9cscrollxe2x80x9d the map display in a direction relative to the heading.
Accordingly, the present invention provides a unique navigation device which combines data from a plurality of sensors and position information obtained from GPS, and thus provides the user with a single unit that houses most, if not all, of the functions typically desired (or required) for land-based navigation. Additionally, as stated, the simultaneous access to GPS information, compass information, altimeter information, and topographical map data allows for features such as route elevation profiles and moving maps, even when the user is standing still (or moving slowly).